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Joint Passive Time Reversal and Multichannel Equalization for Underwater Communications

J.P. Gomes
ISR - Instituto Superior Tecnico, 1000 Lisboa, Portugal
A. Silva
and S.M.Jesus ,
SiPLAB-FCT, Universidade do Algarve, 8005-139 Faro, Portugal

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Ref.: Proc. OCEANS MTS/IEEE ' 2006, (ISBN:), p., Boston,USA, September 2006. (to appear)

Abstract : This paper presents experimental results for a passive time reversal experiment conducted off the west coast of Portugal during the MREA'2004 mission. A single acoustic projector generated a 2/4-PSK stream at 200 and 400 baud, modulated around a carrier frequency of 3600 Hz. The signals were received at a range of about 5 Km on a vertical array with 8 hydrophones placed at depths of 10, 15, 55, 60, 65, 70, 75, 80 m.
Time reversal - both active and passive - has aroused considerable interest recently in communications applications as a mean of partially compensating for the intersymbol interference (ISI) introduced by the channel at the receiver [1]. The results of several experiments at sea reported in the technical literature suggest that time reversal by itself will not ensure reliable detection of the transmitted symbols, and must be complemented by adaptive equalization at the receiver (see also the theoretical results of [2]). Nonetheless, this computationally inexpensive technique is useful because, when compared with full multichannel equalization, it allows simpler equalizers to be used [3]. Arguably, the resulting overall reduction in complexity at the receiver more than makes up for the moderate degradation in performance.
In a few papers that have been published on simultaneous equalization and time reversal the two systems are operated in tandem, i.e., time reversal creates a single-channel signal which is then independently processed by an equalizer. This configuration is inevitable in active time reversal, where the ocean itself generates the refocused waveform. In passive time reversal, however, the signals received at an array of hydrophones are synthetically combined, and may therefore be individually postprocessed after convolving them with estimates of the time-reversed channel impulse responses.
The analysis of MREA'04 data has shown that, when the synthetically recombined signal exhibits poor signal-to-interference (ISI+noise) ratio, this can often be attributed to the individual multichannel components interfering destructively in spite of a seemingly appropriate temporal alignment. We propose to multiply each of them by an adaptive complex term before summing, so that constructive interference can be restored. The parameters of these rotators are jointly optimized with the downstream adaptive equalizer coefficients at each symbol interval to minimize the output MSE. In the limit when the downstream equalizer has no feedforward filter, the structure can be thought of as a low-complexity multichannel combiner that takes advantage of a useful physical property of sound propagation in the ocean to drastically reduce the number of adaptive parameters to be estimated.
In the MREA'04 setup the 8 unevenly-spaced array sensors are separated by a minimum of 5 m, or about 12 wavelengths at the carrier frequency. Under these conditions, which significantly differ from the classic half-wavelength separation in array processing, the question arises as to whether focusing occurs by coherent beamforming, or mainly as a result of spatial diversity across the array. While that question is not answered thoroughly in the paper, a statistical characterization of this SIMO channel is performed that provides clues on the spatial correlation. Moreover, results are presented for different subarray sizes when studying the performance of the various demodulation structures to assess the impact of the spatial dimension.
In addition to the proposed multichannel receiver structure, our results illustrate the performance of plain time-reversal, time-reversal followed by single-channel adaptive equalization, and multichannel equalization with and without reduced-complexity multichannel combining.

[1] G. Edelmann et al., "An initial demonstration of underwater acoustic communication using time reversal", IEEE JOE 27(3), pp. 602-609, Jul. 2002.
[2] M. Stojanovic, "Retrofocusing techniques for high-rate acoustic communications", JASA 117(3), Pt. 1, pp. 1173-1185, Mar. 2005.
[3] A. Silva, S. Jesus, J. Gomes, and V. Barroso, "Underwater acoustic communications using a 'virtual' electronic time-reversal mirror approach", in P. Chevret and M. Zakharia, editors, 5th European Conference on Underwater Acoustics, pp. 531-536, Lyon, France, June 2000.

ACKNOWLEDGMENT: this work was partially supported by FCT projects NUACE - POSI/CPS/47824/2002 and RADAR - POCTI/CTA/47719/2002.

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